1. Field of the Invention
This invention relates to the field of devices for mechanically tuning thin-film resonators. More particularly, it relates to slidable tuning elements suitable for use in filters maintained at cryogenic temperatures.
2. Description of Related Art
Recently, the field of thin-film, high-Q resonators for microwave applications has advanced rapidly. This advance has been particularly notable in the area of superconductive filters, particularly those designed for communications systems operating at frequencies near 850 MHz (cellular) or 2 GHz (PCS). Typically, the filters are multiple-resonator microstrip line distributed or lumped element L/C circuits. Both bandpass and band stop filters have been described. Examples of these filters may be found, for example, in Zhang, et al., "Frequency Transformation Apparatus and Method in Narrow-band Filter Designs", U.S. Ser. No. 08/706,974; Hey-Shipton, et al. "High temperature superconductor lumped element band-reject filters", U.S. Pat. No. 5,616,539 and Hey-Shipton et al., "High temperature superconductor staggered resonator array bandpass filter", U.S. Pat. No. 5,616,538. Filters with as many as nineteen poles or resonators have been reported.
For satisfactory performance of multiple-resonator filters, each resonator must resonate at the same midband frequency .omega..sub.0. While it is theoretically possible to design a filter in which the resonators each exhibit the same midband frequency, in practice manufactured filters, whether normal metal, dielectric or superconductive, do not necessarily have precisely the frequency response of the design, due to such things as variations in film characteristics, assembly variations and temperature effects. It is well known in the art to tune filters by positioning conductive or dielectric materials in the electromagnetic field above the filter element. For example, Matthaei, et al. "Microwave filters, Impedance-Matching Networks, and Coupling Structures" Artech House Books, MA, 1964 (reprinted 1980), pp. 168-173 discusses in detail the process of tuning a variety of types of bandpass filters.
Typically, designers utilize a screw-type tuning member. However, Superconductive thin-film filters present additional challenges. First, superconductors only exhibit a superconductive state below critical temperatures, which currently requires operating temperatures below about 77K. The filter must be tuned at this temperature and be capable of maintaining that tuning when brought to room temperature for shipping and then recooled for use. Second, for maximum performance these superconductors must be epitaxially deposited on a single crystal substrate. Practically, this limits the size of a filter to about 2-4 inches in any dimension. The filter designer will be faced with the task of positioning as many as 16 filter poles, each requiring tuning, within that limited space. This, in turn, requires densely packed tuning elements.
Prior art tuning mechanisms for thin-film superconductive rf resonators utilized a screw adjustment much like that used in larger, room temperature filters. The tip of the screw could be a conductive, magnetic or dielectric material. To avoid losses, and thus realize the high-Q advantage provided by the use of superconductive inductors, superconductive tuning tips are preferred over normal metal tips. Examples of such tuning methods are described in Higaki, et al., "Microwave Resonator of Compound Oxide Superconductor Material Having a Tuning Element With A Superconductive Tip", U.S. Pat. No. 5,391,543 and in Hey-Shipton et al., U.S. Pat. No. 5,616,538.
Screw-type tuning members, however, have several disadvantages when used in small, cryogenically cooled filter assemblies such as those required for thin-film superconductive filters. The tuning resolution is limited by the pitch of the screw threads. Axial stress tends to cause creep and stress relaxation which can alter critical positioning of the screw after tuning. The screws tend to wobble due to required thread clearances. The screws have large surface areas, leading to high radiative losses. They require relatively large spacing, limiting design choices for the filters. The required machining of the package and the use of multiple parts, make the package costly.